Gnome

When switching workspaces, only first screen switches. To make all screens switched:

 gsettings set org.gnome.shell.extensions.classic-overrides workspaces-only-on-primary false

Graphical tool "dconf-editor" can be used as well. Expand the path "org.gnome.shell.extensions.classic-overrides" and highlight the setting for "workspaces-only-on-primary". Turn off "Use default value" then click on False in the "Custom value" setting. Click the "Apply" button at the top.

To change the number of workspaces: "org.gnome.desktop.wm.preferences.num-workspaces". Set "Use default value" = OFF, than type the value you want.